Graft copolymers made by reacting a dry, liquid polyethylenimine and propylene sulfide at 30.degree. C.-40.degree. C., using from 0.2 to 4 moles of propylene sulfide per repeating ethylenimine unit in the polyamine, are disclosed in Journal of Polymer Science (Polymer Chemistry Edition) 10, 3519-3258 (1972). The resulting graft copolymers have a high mercaptan functionality resulting from graft initiation on various sites of the polyamine. Separation of the graft copolymer from propylene sulfide homopolymer could not be effected because of extremely rapid oxidative cross-linking of the product in air. Cross-linking of the polymers render them insoluble in aqueous systems and therefore their use as aqueous corrosion inhibitors becomes impractical where small amounts of oxygen are present. Storage of such a product requires elaborate care to avoid excessive exposure to air or other oxygen-containing atmosphere.